Implicit learning of artificial grammatical structures after inferior frontal cortex lesions

ObjectivePrevious research associated the left inferior frontal cortex with implicit structure learning. The present study tested patients with lesions encompassing the left inferior frontal gyrus (LIFG; including Brodmann areas 44 and 45) to further investigate this cognitive function, notably by using non-verbal material, implicit investigation methods, and by enhancing potential remaining function via dynamic attending. Patients and healthy matched controls were exposed to an artificial pitch grammar in an implicit learning paradigm to circumvent the potential influence of impaired language processing.MethodsPatients and healthy controls listened to pitch sequences generated within a finite-state grammar (exposure phase) and then performed a categorization task on new pitch sequences (test phase). Participants were not informed about the underlying grammar in either the exposure phase or the test phase. Furthermore, the pitch structures were presented in a highly regular temporal context as the beneficial impact of temporal regularity (e.g. meter) in learning and perception has been previously reported. Based on the Dynamic Attending Theory (DAT), we hypothesized that a temporally regular context helps developing temporal expectations that, in turn, facilitate event perception, and thus benefit artificial grammar learning.ResultsElectroencephalography results suggest preserved artificial grammar learning of pitch structures in patients and healthy controls. For both groups, analyses of event-related potentials revealed a larger early negativity (100–200 msec post-stimulus onset) in response to ungrammatical than grammatical pitch sequence events.ConclusionsThese findings suggest that (i) the LIFG does not play an exclusive role in the implicit learning of artificial pitch grammars, and (ii) the use of non-verbal material and an implicit task reveals cognitive capacities that remain intact despite lesions to the LIFG. These results provide grounds for training and rehabilitation, that is, learning of non-verbal grammars that may impact the relearning of verbal grammars.

研究目标：既往研究将左额下皮层与内隐结构学习建立关联。本研究针对累及左额下回（left inferior frontal gyrus, LIFG，包含布罗德曼44区与45区）的脑损伤患者展开实验，以进一步探究该认知功能，具体手段包括采用非言语材料、内隐测试方法，并通过动态注意强化潜在剩余功能。研究对象与健康匹配对照组均在无告知的内隐学习范式下接触人工音高语法，以此规避语言加工受损带来的潜在影响。

研究方法：研究对象与健康对照组先聆听由有限状态语法生成的音高序列（暴露阶段），随后对全新音高序列完成分类任务（测试阶段）。无论暴露阶段还是测试阶段，参与者均未被告知实验隐含的语法规则。此外，音高结构以高度规律的时间背景呈现——此前已有研究证实时间规律性（如节拍）对学习与感知具有增益效应。基于动态注意理论（Dynamic Attending Theory, DAT），本研究提出假设：时间规律的背景有助于形成时间预期，进而促进事件感知，最终助力人工语法学习。

研究结果：脑电图结果显示，患者组与健康对照组均保留了音高结构的人工语法学习能力。对两组受试者的事件相关电位分析均发现，相较于符合语法的音高序列事件，不符合语法的序列会引发更强的早期负波（刺激起始后100–200毫秒）。

研究结论：上述发现表明：（1）左额下回并非人工音高语法内隐学习的专属脑区；（2）采用非言语材料与内隐任务可揭示，即便左额下回存在损伤，受试者仍保留完整的认知能力。本研究结果为训练与康复提供了理论依据，即对非言语语法的学习或可助力言语语法的重学习。